The present invention relates to a method of printing an image on an article such as a container or a wrapper for a product or a package.
In the description and in the following claims, the term "image" means, in a broad sense, a text, a drawing, a logo, a bar code, or any other two-dimensional graphic representation.
The application of codes (for example bar codes) and/or images of various types to wrappers for products in order to identify the product and/or to indicate, for example, its sale price and possibly other information is a widespread practice in industry and commerce.
Adhesive labels on which the codes and/or images have previously been printed are often used for this purpose.
This solution has various disadvantages from both economic and aesthetic points of view. Moreover, the labels may become detached or may be removed from the products, which is a further problem.
In order to code articles such as containers for products, it has also been proposed to pre-print the texts and/or codes onto the containers before the product is packaged (for example, by typography or silk-screen printing).
This solution has the disadvantage that it requires the provision, and hence the ORGANIZATION, of stocks of pre-printed containers or wrappers and may even involve considerable wastage when it is necessary to change the code associated with a particular article for which the container or wrapper is intended.
It has also been proposed to print codes and/or images of other types directly onto products which have already been wrapped, by the thermal transfer of ink from an inked ribbon.
If the shape and consistency of the packaged product allow, it is possible to use thermal transfer devices with printing blocks for this purpose. In order to print various images or codes, this solution involves the provision and ORGANIZATION of a corresponding set of printing blocks, since it is not possible to modify a printing block in real time during production. Moreover, it is not possible to use dot-line or dot-matrix printing heads, which are typically made of ceramic material, and hence are rigid, when the packaged product is of irregular shape and/or consistency or when the surface which is to receive the print is quite rough.
German patent DE-A-39 35 345 describes a printing method. In the method described in this document, the positive image is formed (possibly as a mirror image) on a portion of an auxiliary flexible ribbon by the thermal transfer of some of the ink carried on one face of an inked ribbon, by means of a dot printing head. The positive image thus formed is then transferred from the auxiliary ribbon to an article by means of a transfer device with a heated pad.
This method has various disadvantages.
In the first place, it can be carried out only with the use of an auxiliary ribbon which makes the equipment used more expensive and bulky.
Moreover, the transfer of the image onto the article necessarily takes place by means of a double transfer of ink, that is, from the inked ribbon to the auxiliary ribbon and then from the latter to the article.
In this connection, it should be noted that the most usual and economically most suitable inked ribbons typically comprise a substrate ribbon, for example, of mylar, on one face of which a layer of hot-melting (typically resin-based) ink is deposited with the interposition of a layer of a release agent. A film of a hot-melting adhesive, for example of the so-called "hot melt" type, is also deposited on top of the layer of ink.
With the use of an inked ribbon of this type, in the method according to DE-A-39 35 345, the ink transfers well from the inked ribbon to the auxiliary ribbon, but transfers badly from the auxiliary ribbon to the article. In fact, as a result of the first transfer, the ink is fixed firmly to the auxiliary ribbon by virtue of the adhesive which covers the layer of ink on the inked ribbon. The ink transferred to the auxiliary ribbon, however, is no longer covered by adhesive but, on the other hand, may have traces of release agent. The ink cannot, therefore, be anchored perfectly to the surface of the article during the second and final transfer. This may involve a decline in the quality of the image printed on the article and/or an image which is not sufficiently stable.